Since 1990, the wired local area network defined by IEEE (The Institute of Electrical and Electronics Engineers, Inc.) standard 802.3 10BASE-T, an acronym which stands for 10 Mbit/s, baseband transmission, twisted-pair wiring, has rapidly become preeminent. In addition to its widespread use in new installations, it has, in many cases, supplanted earlier coaxial cable Ethernet and Token-Ring installations. The number of 10BASE-T installations is already large and continues to grow rapidly.
10BASE-T network stations are each connected to a repeater via a duplex, unshielded twisted-pair cable. The repeater accepts a transmitted signal from one station and repeats it to the receivers of all the other stations. This topology facilitates changes and fault location, and the unshielded twisted-pair cable is inexpensive and easy to co-install with telephone wiring. Sixteen-port repeaters of a basic design are easily affordable by small businesses. A local area network repeater may also be connected to a metropolitan area network, wide area network, the Internet, and the proposed "information highway".
Prior to 1993, most stations connected to the 10BASE-T network were desktop personal computers; however, it is becoming increasingly common for portable computing units to be equipped with accessory slots conforming to the PCMCIA standard which can accept credit card-sized 10BASE-T adapters. The users of portable computing units would receive significant benefit from a means for extending the 10BASE-T network to establish communication ad hoc among collaborators on a common task, between desktop and portable computers, or with the wired network. Two 10BASE-T stations can form a "network" using a cross-connected cable in which the transmitter of the first station is connected to the receiver of the second station and vice versa. However, since n stations require [n/2].multidot.[n-1] duplex twisted-pair cables, it is impracticable for a multi-station ad hoc network to be connected in this manner.
The prior art includes a variety of IR, or infrared, and RF, or radio frequency, systems. Prior-art IR systems are limited to less than 10 Mbit/s by either the choice of modulation technique or receiver design limitations or are so directive that it would be difficult or impossible to form a collaborative network of portable computers. Prior art RF systems are either limited by spectrum availability to much less than 10 Mbit/s, or designed for stationary mounting and alternating current powered operation.
The environment of a 10BASE-T network extension is local, which minimizes the range and coverage requirements, as well as the likelihood of "hidden" stations. Thus, either IR optical or microwave radio frequencies are appropriate to convey the Ethernet signal by on-off keyed modulation. On-off keyed modulation is spectrally efficient and simple to generate and detect. Signal emission in an IR optical receiver may employ one or more light-emitting diodes, such as the Stanley Electric Co., Ltd. DN304, or laser diodes, while signal emission in a microwave RF transmitter may employ a Gunn diode of the type used in law-enforcement speed detectors. Signal detection in an IR optical receiver may employ one or more photodiodes, while signal detection in a microwave RF receiver may employ a Schottky barrier diode. Information transmitted using IR optical signals is inherently private and resistant to interference since most building materials are opaque. In addition, the generation of IR optical signals does not require a license, and safety and international spectrum regulation issues are avoided.
The CSMA/CD, or carrier sense, multiple access with collision detection, protocol specified by the IEEE 802.3 Ethernet standard provides for greater network throughput by quickly aborting transmissions involved in collisions, or simultaneous transmissions by two transmitters. The protocol determines that a collision has occurred when a station detects the presence of a valid signal on its receiving pair during the time that its transmitting pair is active. In the wired 10BASE-T environment, it is certain that the colliding signal originates from a second station. However, if the transmitting and receiving pairs of each station are not connected to the repeater but to a wireless emitter and receiver, respectively, then the wireless receiver will often respond to its own emitter due to reflection from nearby objects or surfaces. Under this condition, the 10BASE-T hardware and protocol will identify every transmission as a collision, the collision protocol will be continuously engaged, and the network will become inoperative. Thus, standard 10BASE-T Ethernet hardware and protocol is inadequate when the wire transmission medium is replaced with a wireless transmission medium.
The 10BASE-T Ethernet protocol also provides for verifying the integrity of the twisted-pair connection between the network station and repeater by sending link integrity pulses having a duration of 100 nanoseconds at 16 millisecond intervals during the time when no data is being transmitted. If neither a data packet nor a link integrity pulse is received for a typical duration of 110 ms, then the protocol enters the link test fail state, and the transmit and receive functions are inhibited. When either a data packet or, typically, four consecutive link integrity pulses are received, then the Ethernet adapter will exit the link test fail state and re-enable the transmit and receive functions. Since a wireless receiver may not respond reliably to an isolated 100 nanosecond pulse, and many Ethernet adapters do not provide for disabling the link integrity test function, the 10BASE-T Ethernet link test will inhibit operation of the network if it is used in the wireless environment.
Accordingly, there is a need for a wireless method, transceiver and system which is compatible with a standard, twisted-pair 10BASE-T Ethernet interface or card so that the wired and wireless environments can be made equivalent. If such a wireless method, transceiver and system were available then file-sharing and communications capabilities could be user-defined by the same operating system or networking software in either the wired or the wireless environment. Moreover, wireless access to the wired 10BASE-T local area network would be greatly simplified since no protocol or data rate translation would be required.